Support default values for attributes

[chasefinch]

Would be good to have some attributes allow a default value that's set when the attribute name is omitted. This might mean that the syntax for "is_unique" needs to be more explicit. For example:

class Currency(DataEnum)
    data_attributes = (
        ('code', dict(is_unique=True)),  # Simple syntax ('code', True) would also still be allowed
        ('symbol', dict(default='$')),
    )

Currency.CAD = Currency(code='CAD')  # Uses default symbol '$'